1. Field of The Invention
This invention relates to a process for the production of fatty acid alkyl esters, particularly methyl esters, from natural fats and oils containing free fatty acids by catalytic transesterification.
2. Description of Related Art
Fatty acid methyl esters have acquired considerable commercial significance as starting materials for the production of fatty alcohols and other oleochemical products, such as ester sulfonates, fatty acid alkanolamides and soaps. On an industrial scale, fatty acid methyl esters are mainly produced by catalytic transesterification (alcoholysis) of fatty acid triglyceride mixtures of the type present in fats and oils of vegetable and animal origin.
Natural fats and oils almost always contain considerable quantities of free fatty acids. Their content of free fatty acids varies over a wide range, depending on the origin of the material and its previous history, and almost always exceeds about 3% by weight.
Various processes are available for the transesterification of naturally occurring fatty acid triglycerides with alcohols. The choice of process conditions depends to a large extent upon the quantity of free fatty acids present in the triglyceride mixture.
Atmospheric transesterification of fats and oils to form the corresponding fatty acid ester mixtures may be effected with a 0.5 to 1.0-molar excess of alcohol in the presence of an alkali catalyst under atmospheric pressure and at temperatures of 25.degree. to 100.degree. C. Such a process is described in U.S. Pat. No. 2,360,844 as the first stage of a soap manufacturing process. This alkali-catalyzed, atmospheric transesterification process may be carried out without any problems as long as the starting materials used are fats and oils which are substantially free from water and which have a free fatty acid content of less than 0.5% by weight (corresponding to an acid number of about 1).
Fats and oils having a relatively high content of free fatty acids may be transesterified in a high pressure transesterification process with a 7- to 8-molar excess of methanol in the presence of alkali or zinc catalysts to form the corresponding fatty acid methyl esters. This process is carried out at a temperature of 240.degree. C. and at a pressure of about 100 bar. (Ullmann, Enzyklopadie der technischen Chemie, 4th Edition, Vol. 11 (1976), page 432).
Compared with high-pressure transesterification, atmospheric transesterification uses considerably less methanol and, by virtue of the lower reaction temperatures, less energy. In addition, atmospheric transesterification does not require expensive pressure reactors. Commercial grade fats and oils, however, almost always contain relatively large quantities of water and fatty acids. As a result, atmospheric transesterification of these commercial mixtures requires preliminary drying and a reduction in the acid number, for example by conversion of the free fatty acids into the corresponding alkyl or glycerol esters in a pre-esterification reaction. Pre-esterification of the acid-containing fats and oils may be carried out in the presence of alkaline catalysts at temperatures of 240.degree. C. and at pressures of 20 bar. (Ullmann, Enzyklopadie der technischen Chemie, 4th Edition, Vol. 11 (1976), page 432). This method of pre-esterification with methanol also requires the use of expensive pressure reactors.
An object of the present invention is to facilitate the production of fatty acid esters, particularly methyl esters, from triglyceride starting materials containing relatively large quantities of water and free fatty acids.